As well known, optical add/drop multiplexer (OADM) devices, enabling the flexible insertion (add) or extraction (drop) of a specific wavelength in optical fiber communications, have been indispensable components for wavelength division multiplexing (WDM)-based networks, accommodating large bandwidths for the global spread multimedia communications. These components not only allow the extraction of a wavelength from a transmission loop and the addition of the same wavelength to the network, but also, monitor the signals in transparent networks to identify and locate the possible failures. Numerous different architectures of OADMs, based on different optical devices have been revealed, such as array waveguide gratings, fiber-based, and integrated optics-based devices. Among them, the integrated optics-based devices attract more attention for their compactness, mechanical stability and suitability for mass-production, as well as the inclusion of several functions on a single chip. These include micro-ring resonators, Mach-Zehnder interferometer (MZI) based add/drop filters, grating-assisted co-directional couplers, asymmetric Bragg coupler (ABC) based filters, and Bragg reflector channel waveguide filters. Micro-ring resonators have been the promising devices and used as passive and active components due to their unique properties. However, they generally suffer from either a narrow free spectral range (FSR), or an expensive fabrication equipment, that restrict its application range in WDM networks. The merits and drawbacks of the other devices have been depicted in the present inventor's previous paper (W. C. Chuang, A. C. Lee, C. K. Chao, and C. T. Ho, “Fabrication of optical filters based on polymer asymmetric Bragg couplers,” Opt. Express 17, 18003-18013 (2009)). The performance and characteristics of ABC-based filters, based on a single grating in one arm of a non-resonant coupler and operated in a contra-directional mode, have been examined in detail.
In integrated optics or guided wave optics, high quality and inexpensive materials are required for highly-integrated photonic processors. Polymeric materials possess unique optical and mechanical properties such as relatively low refractive index resulting in lower surface-roughness scattering, easily manipulating by conventional or unconventional fabrication technologies, providing excellent platform for integrating numerous materials with different functions, and high flexibility for being bent and attached to non-planar surface. Additionally, they are cost-effective and reliable for mass-production. Polymer surface-relief Bragg grating, which provides a narrow bandwidth, low crosstalk, and flat-top pass band, has become an essential component for various applications in optical communications and optical sensing. For example, J. Kang et al demonstrated a narrow band filter of 0.2 nm bandwidth using polymer surface-relief Bragg grating on an integrated optical waveguide. R. Horvath et al fabricated a cost-effective polymer waveguide sensor chip using polymer surface-relief Bragg grating integrated on polymer film as a light coupler.
In the past, we demonstrated a process to rapidly produce submicron range gratings on a polymeric waveguide for optical filters. A high aspect ratio and vertical sidewalls are obtained, and consistent reproduction of the grating on a UV polymer has been achieved.
Recently, the inventor of the present inventor combined the holographic interferometry, soft lithography, and a simple replication processes for fabricating a polymeric ABC filter. The method includes the following procedures. The grating structure on a polymer was first fabricated using holographic interferometry and the micro-molding processes. An ABC filter was produced by a two-step molding process where the master mold was first formed on negative tone photo-resist and subsequently transferred to a PDMS mold. The PDMS silicon rubber mold was used as a stamp to transfer the waveguide coupler pattern of polymeric ABC filter onto a UV cure epoxy. Narrow bandwidths and deep transmission dips were obtained. However, the device has a disadvantage that the gratings were engraved concurrently on the bottoms of a pair of dissimilar waveguides, embedded into a planar substrate, and therefore an undesired reflection wavelength, denoted by self-reflection Bragg wavelength, caused by the grating of input waveguide was occurred in the input end. In order to overcome the above drawback, we develop a process, incorporating the above technologies with capillary effect and microscopy technologies to fabricate an ABC filter without any self-reflections.
Polymeric ABC filters were constructed using the planar channel waveguide configuration. A pair of parallel channel waveguides with different widths was proximally embedded into a planar substrate. These two waveguides are asynchronous because the effective indices of the two waveguides are quite different. In spite of the large index mismatch between the two waveguides, an efficient power coupling was achieved using the Bragg grating, engraved on the bottom of the either/both waveguides. Due to the close proximity of the two waveguides, engraving the grating on the bottom of the either waveguide is much more difficult than that of the both ones. However, if the gratings are engraved on the bottoms of both waveguides, the maximum self- and cross-reflection power coupling simultaneously occurred in the input and drop ports, respectively. The self-reflection light results in broadening the transmission spectrum of the filters because of the spectral overlapping with the cross-reflection one. In the previous work, we made the two decoupled waveguides quite dissimilar to avoid the spectrum overlapping. In this present invention, we develop a process, comprising of capillary effect and microscopy technologies, to eliminate the self-reflection light by removing the grating of the input waveguide; the schematic diagram of an ABC-based polymeric filter is depicted in FIG. 5. It is noted that there is no grating on the input waveguide. To our best knowledge, it is the first report for ABC structures with a single-grating waveguide on polymeric materials.